The drive unit of motor vehicles (engine and power transmission train) is increasingly subjected to regulating and control mechanisms in order to optimize the interplay of an engine and its associated transmission; thus, for instance, to reduce fuel consumption in motor vehicles it will be necessary not only to improve engine control systems but also to adapt transmission ratios. To achieve this, automatic transmissions present themselves as a solution, with CVTs (continuously variable transmissions) in particular being increasingly used; this type of transmission allows continuous transmission adjustment until the required transmission ratio has been set. By means of a transmission element (chain, band, belt, etc) located between the discs of two disc assemblies, the required transmission ratio can be continuously adapted: by applying the same contact pressure values on these two disc assemblies--this corresponds to the so-called pretensioning force--the force level for this transmission element will be adjusted; specifying different values for this contact pressure on the two disc assemblies will create an additional transport force component for moving this transmission element; by varying this contact pressure, and thus the transport force, the transmission ratio may be continuously varied in any number of steps.
The regulating processes employed in continuously variable transmissions normally use transmission or speed controllers to effect control. The transmission input or drive speed will be controlled by means of a control circuit designed as a speed controller, this transmission input or drive speed represents the product from transmission output speed, which is proportional to vehicle velocity, or drive speed (disturbance variable), and transmission ratio (control variable); the regulating processes incorporate adaptive strategies to adapt characteristic control fields to the type of driver on the one hand ("long-term strategy: engine performance required by the respective driver"), or to a given environment ("medium-term strategy": for example, slip detection, towing a trailer, climbing a hill), and, finally, to the current driving situation ("short-term strategy": for example, overtaking another vehicle). Furthermore, in order to improve control response, the speed controller can be provided with (post-connected to) a second control circuit of acceleration control type, where the time derivative (gradient) of the transmission input speed represents the controlled variable; that is, in addition to speed control, a speed gradient control will also be effected. The disadvantage of these regulating processes for controlling continuously variable transmissions is that it is not possible to effect (even though this might often be desirable) a control, limitation, or suppression of the control behavior in respect of the control variable transmission ratio, as only the controlled variable transmission input speed is monitored to provide a measure for the transmission ratio: for instance, it is possible that during acceleration or deceleration of the vehicle an unintended change in the transmission ratio, due to the underlying speed gradient control, and thus a change in the transmission settings can occur; in particular, this can lead to an unintentional movement into the so-called mechanical end position of the transmission where strength and/or service life of the transmission element may be detrimentally affected or where the transmission element can be destroyed.